Exhaled nitric oxide stability over two years in relation to COPD outcomes

The non-invasive biological marker exhaled nitric oxide (FE_NO) is increasingly used in asthma, but its clinical role in COPD is less established. FE_NOhas been reported to be both high and low outside the COPD exacerbation period. The study aimed to follow FE_NOvalues over two years during stable conditions in a cohort of COPD subjects participating in the TIE-study (Tools Identifying Exacerbation). The follow-up study included 353 subjects who attended three visits one year apart. The subjects that were ex-smokers (n= 265) had higher FE_NO,50values (median and IQR) compared with current smokers (n= 88), at inclusion 15 (10, 24) versus 9 (7, 15) ppb, after one year 15 (10, 24) versus 10 (7, 18) ppb, and after two years 14 (9, 22) versus 10 (7, 17) ppb, allp< 0.001. All subjects were further divided into two FE_NOgroups: <20 ppb (72%) and ⩾20 ppb (28%). After one year, 81% of the participants remained in the low group and 65% in the high FE_NOgroup. After two years, 71% remained in the low group and 52% in the high FE_NOgroup. The persistent low FE_NOgroup had statistically significantly lower FEV₁%pred and FVC%pred compared to the high FE_NOgroup for all three visits. Among the ex-smokers, the proportion of subjects reporting dyspnoea (mMRC ⩾ 2) was higher in the persistent low FE_NOgroup than in the persistent high FE_NOgroup at all three visits. In conclusion, good consistency in FE_NOover two years is promising for monitoring FE_NOduring stable disease. COPD subjects with persistent low FE_NOhad poorer lung function and reported more dyspnoea than subjects with persistent high FE_NO.